Common aperture antenna

ABSTRACT

This invention relates to antennas ( 26, 28, 30 32, 34 ) including an integrated array of antenna elements ( 36 ). More particularly, the invention relates to antennas ( 26, 28, 30 32, 34 ) in which the array of antenna elements ( 36 ) can be reconfigured to suit a multitude of system functions, such as radar, electromagnetic warfare (EW) and communication. Such antennas ( 26, 28, 30 32, 34 ) are often referred to as ‘common aperture antennas’ and find use on many platforms including airborne vehicles, ships and boats. An antenna ( 26, 28, 30 32, 34 ) is provided that comprises a plurality of antenna elements ( 36 ), the antenna ( 26, 28, 30 32, 34 ) being operable with sets of the antenna elements ( 36 ) organized into first order groups ( 14, 46 ) and with sets of first order groups ( 14, 46 ) organized into sets of second order groups ( 18 ).

This application is the U.S. national phase of international applicationPCT/GB03/02552, filed in English on 13 Jun. 2003, which designated theU.S. PCT/GB03/02552 claims priority to GB Application No. 0213976.4filed 18 Jun. 2002. The entire contents of these applications areincorporated herein by reference.

This invention relates to antennas comprising an integrated array ofantenna elements. More particularly, the invention relates to antennasin which the array of antenna elements can be reconfigured to suit amultitude of system functions, such as radar, electromagnetic warfare(EW) and communication. Such antennas are often referred to as ‘commonaperture antennas’ and find use on many platforms including airbornevehicles, ships and boats. In addition, this invention relates to anantenna system comprising a plurality of such antennas and to platformscomprising such an antenna or antenna system.

Generally, such common aperture antennas receive and transmit radiowaves over a wide range of frequencies. The antenna architecture mustperform a combination of radio frequency (RF) and optical beam-formingfunctions, such that each of the system requirements can be met. Forexample, electronic surveillance measures (ESM) relies on the analysisof multiple beams whereas communication generally only requires a singlebeam to be transmitted or received.

Over recent years, the concept of aperture integration where manyfunctions are performed by a common aperture, rather than using separateantennas for each function, has been considered and the following is alist of some of the potential benefits:

-   improved integration of different functions, such as radar and    communication;-   reduced blockage problems between operation of different antenna    requirements;-   reduction of radar cross section (RCS);-   better use of antenna positional and volume constraints including    reduced weight and reduced drag; and-   reduced costs to build and maintain.

However, in order to realise these potential benefits the followingproblems need to be solved:

-   how to amalgamate all the beam-forming requirements of the many    diverse functions into a single architecture;-   how to amalgamate in a cost effective way that minimises the amount    of hardware duplication;-   how, to incorporate the required flexibility into the architecture    that allows rapid selection of any of the required system functions;-   how to share simultaneously the aperture between as many functions    as possible;-   how to enable digital signal processing to be used over a wide    frequency bandwidth, within the constraints of available analogue to    digital (A/D) devices;-   how to operate over a wide frequency bandwidth with most functions    only requiring a narrow instantaneous bandwidth; and-   how to manage resource sharing between transmit and receive    functions.

Two papers that have discussed the concept of aperture integration areMultifunction Wide-Band Array Design by Hemmi et al (IEEE Transactionson Antennas and Propagation, 1999, volume 47, pages 425 to 431) andOverview of Advanced Multifunction RF Systems by Hughes and Choe(International Symposium on Phased Array Systems and Technology, 2000,pages 21 to 24). Their work uses wide frequency bandwidth radiatingelements in the antenna array that operate over the frequencies requiredby all the combined system functions. The beam-forming is performed byusing separate dedicated RF beam-forming networks for each function. Thevarious functions are utilised by selecting the appropriate beam-formingnetwork by means of RF switching circuits.

However, providing a dedicated beam-forming network for each function isnot only very costly but can prove impractical to implement. In atwo-dimensional array, the front-end electronics associated with eachantenna element must be packaged in a tube with the cross-sectional areaof the element's unit cell. Worse still, multiple beams requireduplication so that the active electronics must be miniaturised further.Where this is difficult to implement for receive functions, thechallenge is far greater for transmit functions where heat dissipationbecomes a critical factor.

From a first aspect, the present invention resides in an antennacomprising a plurality of antenna elements, the antenna being operablewith sets of the antenna elements organised into first order groups andwith sets of first order groups organised into sets of second ordergroups.

Optionally, the organisation of antenna elements into first order groupsis fixed. Hence, the controller has a fixed arrangement of antennaelements with which to work. The controller may further comprise acontroller operable to reconfigure dynamically the organisation of firstorder groups into second order groups.

Optionally, the antenna further comprises a first beam forming networkoperable to receive signals from the antenna elements and/or operable totransmit signals to the antenna elements, wherein the first beam formingnetwork comprises a local network for manipulating signals received byor to be transmitted by an antenna element and a remote network formanipulating the signals received from or to be transmitted to aplurality of the local networks. Advantageously, all local networks areconnected to a single remote network. Preferably, the signals from theelements of a first order group are combined within the local networkbefore transmission to the remote network or a signal from the remotenetwork is separated within the local network for transmission to theelements of a first order group.

The local network may be operable with RF signals and, optionally, theremote network may be operable with optical frequency signals. Whereboth of these options are combined, it is advantageous for the localnetwork to be operable to upconvert an RF signal to an optical frequencysignal prior to transmission to the remote network. Preferably, theremote network is operable to digitise a signal received from the localnetwork. In some applications, it is beneficial for the remote networkto be operable to provide true time delay.

Optionally, an antenna element is operable with two polarisations. Bythis, it is meant that either a single radiating element is able totransmit and receive two polarisations or that two radiating elementsare grouped together as an ‘antenna element’, each radiating elementbeing operable with a different polarisation. Advantageously, thepolarisations are mutually orthogonal.

In a preferred embodiment, each second order group is provided with itsown receiver.

Optionally, the antenna comprises at least one group of antenna elementsfor use in ESM analysis mode. Moreover, the antenna may further comprisea second beam-forming network operable to receive signals from theantenna elements of the at least one group of antenna elements for usein ESM analysis mode. Advantageously, the second beam-forming networkcomprises a local network and a remote network. The local network may beoperable with RF signals and, optionally, the remote network may beoperable with optical frequency signals. Optionally, the local networkis operable to upconvert the RF signal to optical frequencies prior totransmission to the remote network. Where the antenna element isoperable with two polarisations, it is convenient for the local networkto upconvert the RF signal from each polarisation to optical frequenciesand then to transmit separately the optical signals to the remotenetwork.

Optionally, the antenna comprises ESM elements for transmission of ESMsignals.

The invention also extends to an antenna system comprising a pluralityof antennas as described herein above. Furthermore, the invention alsoextends to a platform comprising an antenna as described herein above.By platform, it is meant any host for the antenna or antenna system.Hence, a platform may be a building or other similar structure (such asa mast) or any type of vehicle (such as land vehicles, airborne vehiclesor waterborne vehicles).

In order that the invention can be more readily understood, referencewill now be made, by way of example only, to the accompanying drawingsin which:

FIG. 1 is a schematic representation of an antenna system comprising agroup of antenna arrays provided on an airborne vehicle according to afirst embodiment of the invention;

FIG. 2 is a block diagram of the beam forming networks of the firstembodiment;

FIG. 3 is a schematic representation of the architecture of the rightarray of the first embodiment;

FIG. 4 is a block diagram of the multifunction beam-forming network,simplified in that it shows only a single antenna element within theright antenna array; and

FIG. 5 is a block diagram akin to FIG. 4, but this time showing themultibeam ESM beam-forming network.

An example of a dynamically reconfigurable, common aperture antennasystem 10 will now be described. The antenna system 10 provides a meansof electromagnetic beam-forming for a wide variety of operational modes,such as ESM, radar, communication and electromagnetic warfare (EW). Thebeam-forming architecture achieves this over a wide frequency bandwidth,for a wide field of view (sometimes referred to as a field of regard)and for alternative polarisation states.

Previously, it has been proposed to use a dedicated beam-forming networkfor each function. An alternative approach is considered here where acommon beam-forming network is used for all but the ESM analysis mode.This is the only mode that specifically requires multiple simultaneousbeams. A hybrid approach is proposed for the beam-forming network forthe remaining functions. Amplitude and phase control is provided atantenna element level by a local RF network 12. On receive, the signalsare then combined into true time delay (TTD) subarrays 14, upconvertedto optical frequencies and relayed to a remote beam forming network 16that is common to all functions other than ESM analysis. The remote beamforming network 16 combines the TTD subarrays 14 into larger digitalsubarrays 18, each of which has its own receiver 20. The signals arethen separated into individual frequency bands and fed into an AIDdevice 22 and into the digital signal processor (DSP) 24. The antennasystem 10 functions in a similar way on transmit, but signals propagatein the reverse direction.

The proposed architecture solves the problems summarised above byutilising a mixture of RF and optical beam-forming techniques. Moreover,it uses a hierarchy of subarrays that enable the beam-forming to besplit into local and remote functions. This allows much of thebeam-forming to be performed remotely using a common beam-former withoutthe need to duplicate equipment for every single function.

The present invention can be employed in many types of platforms,including airborne vehicles, ships and boats, and the array concept canbe applied to either naval or airborne system mode requirements. It willbe readily appreciated that the proposed beam-forming architectures aregeneric to all these systems. The embodiment of the present invention isdescribed with respect to an airborne application. Specifically, anantenna system 10 mounted on an aircraft is shown in FIG. 1 comprisingleft 26 and right 28 antenna arrays mounted on respective wings of theaircraft, top 30 and bottom 32 antenna arrays mounted on the fuselage ofthe aircraft and a rear antenna array 34 mounted on the tail portion ofthe aircraft.

Ideally, the antenna system 10 should be capable of performing allradar, EW, and communication functions. Some of these functions willhave conflicting requirements for their field of view. For example,search, tracking, radar classification, ground mapping, terrainfollowing and, for the most part, ESM and electronic counter measures(ECM) need to be forward looking and are ideally suited to an antennaarray either within the nose cone of the aircraft or inside the wingedge (as in the present embodiment). However, ESM and ECM also need tobe rear looking. Moreover, synthetic aperture radar (SAR) and groundmoving target indicator (GMTI) radars need to look both sideways anddownwards. Both the satellite and data links used in communications arelikely to require full hemispherical coverage. There is also a need toincrease the field of view of the forward-looking functions beyond ±60°out to possibly ±120° or so. This can either be achieved by using one ormore conformal antenna arrays or by using a plurality of planar arrayslooking in appropriate directions. Alternatively, a mechanically steeredactive array may be used if the overall system time management allows.Accordingly, the SAR and GMTI modes are ideally served by using theantenna arrays mounted along the fuselage, the satellite link mode bythe array on top of the aircraft and the data link mode by the antennaarrays on the top and bottom of the aircraft (or at the rear of theaircraft for back to base transmission). As will now be understood,choice of the numbers and location of the antenna arrays can de variedin accordance with reference to the functions the antenna system 10 isto provide, without departing from the scope of the present invention.

FIG. 2 shows the beam forming networks of each antenna array 26, 28, 30,32, 34 of the present embodiment as a block diagram. As can be seen, thebeam-forming networks controlling the antenna elements 36 within eachantenna array 26, 28, 30, 32, 34 vary between the different antennaarrays of the present embodiment. This is because only the left andright antenna arrays 26, 28 provide ESM functionality and so only theleft and right antenna arrays 26, 28 require dedicated ESM beam-formingnetworks. That said, any of the remaining antenna arrays 30, 32, 34could have ESM functionality.

However, every antenna array 26, 28, 30, 32, 34 has its own localnetwork 12 to provide all functionality other than ESM. In thisembodiment, each local network 12 is identical. This need not be thecase where, for example, not all antenna arrays 26, 28, 30, 32, 34 offerthe same functionality. The local network 12 would be located close tothe array face 48. The left and right antenna arrays 26, 28 that alsoperform ESM have two local networks: one dedicated to ESM analysis 40,the other for all other functions 12. The local networks 12, 40 feedinto one of two remote beam-forming networks; either the ESM multiplebeam remote network 42 or the multifunction remote network 16.

The multiple beam remote network 42 produces simultaneous multiple beamsover the entire frequency band of the antenna system 10. Themultifunction remote network 16 produces single beams over a limitedinstantaneous bandwidth, but is designed to operate over the entirefrequency band of the antenna system 10. The multifunction remotenetwork 16 can be switched to operate in transmit or receive mode. Inits highest gain mode, where a full antenna array 26, 28, 30, 32, 34 isused, beams can only be generated in one direction at a time. For lowergain modes, the antenna elements 36 that make up the antenna array 26,28, 30, 32, 34 can be shared between functions. Within the subarrayconstraints, the antenna array 26, 28, 30, 32, 34 can be dynamicallyreconfigured to dedicate different parts of the antenna array 26, 28,30, 32, 34 to different functions. This allows beams to be formedsimultaneously in different directions and also allows different antennaarrays, such as the left and right antenna arrays 26, 28, to transmitand to receive simultaneously. The networks required to do this aredescribed in more detail below.

The location of the remote networks 16, 42 is not critical to theinvention and can very according to how circumstances dictate. Forexample, in some instances it may be best to locate the remote networks16, 42 centrally, some distance from all of the antenna arrays 26, 28,30, 32, 34. Alternatively, in other instances it may be better to locatethe remote networks 16, 42 next to one of the antenna arrays 26, 28, 30,32, 34. Hence, ‘remote’ should be construed accordingly in that thenetworks need not be distant from all of the antenna arrays 26, 28, 30,32, 34 and may be in close proximity to one or more antenna array 26,28, 30, 32, 34 . It should also be noted that the multiple beam remotenetwork 42 and the multifunction remote network 16 need not be locatedtogether.

FIG. 3 shows schematically the architecture of the right array 28. Aswill be appreciated, the right antenna array 28 is comprised of amultitude (typically thousands) of individual antenna elements 36 thatfill the area within the array boundary 48. Only a small number ofantenna elements 36 are shown in FIG. 3. In addition, dedicatedwide-band ESM elements 44 are shown outside the main antenna array 28.These are used in ESM transmit mode (rather than the receive analysismode for which ESM subarrays 46 are used, as described below) and wouldcover a much wider frequency bandwidth than the antenna array 28. EachESM element 44 produces a single, wide beam and does not require abeam-forming network. These have been included for the sake ofcompleteness and are not discussed further. The architecture of the leftantenna array 26 corresponds to that shown for the right antenna array28 in FIG. 3. The top 30, bottom 32 and rear 34 antenna arrays aresmaller in size but essentially have the same architecture as the left26 and right 28 antenna arrays.

The right antenna array 28 is divided up into subarrays. There are threedifferent types of subarray shown and they will be referred to as TTD14, digital 18 and ESM 46 subarrays.

The antenna elements 36 are divided into hexagonally shaped groups toform a number of TTD subarrays 14. Hexagons have been chosen in thisembodiment due to their close-packing nature, but other shapes such assquares, rectangles and triangles are equally employable. The maximumnumber of antenna elements 36 that can be grouped into the TTD subarrays14 is dependent on the maximum scan range and the instantaneousbandwidth required: the wider the scan range and instantaneousbandwidth, the smaller the TTD subarray 14 must be to ensure undesirablegrating lobes are sufficiently suppressed. The antenna elements 36comprising one of the TTD subarrays 14 is shown at 14′. The division ofantenna elements 36 into TTD subarrays 14 is fixed in this embodiment,although the division can be flexible if required. This latter optionallows the antenna elements 36 to be dynamically reconfigurableaccording to any particular function's needs.

The large number (typically hundreds) of TTD subarrays 14 are arrangedinto digital subarrays 18. Hence, the TTD subarrays 14 correspond to afirst order group and the digital subarrays 18 correspond to secondorder groups. The arrangement of TTD subarrays 14 into digital subarrays18 is flexible in this embodiment allowing dynamic reconfiguration.However, a fixed arrangement could be used if required, although thiswould be to the detriment of flexibility. Each digital subarray 18combines a number of TTD subarrays 14, which are then fed into the A/Ddevice 22 so that digital control can be applied at this level. A majorbenefit of grouping TTD subarrays 14 together in these larger digitalsubarrays 18, is the minimisation of the number of A/D devices that arerequired for the antenna system 10. In fact, as will be described inmore detail later, the TTD subarrays 14 are part of the local network 12whilst the digital subarrays 18 from all antenna elements 36 are handledcentrally by the remote multifunction network 16.

The ESM subarray 46 is used to provide the ESM analysis mode. Individualantenna elements 36 are grouped together to form each ESM subarray 46within the ESM local network 40, which is then fed into the ESM multiplebeam remote network 42. Each ESM subarray 46 operates over the fullfrequency bandwidth of the radiating antenna elements 36 and forms asimultaneous fan of beams in a single plane (although additional ESMsubarrays 46 can be used to provide a fan of beams in orthogonal planes,as shown at 46 in FIG. 3). Individual antenna elements 36 could becombined in this plane prior to the multibeam remote network 42 if anarrower beamwidth is required in this plane. If simultaneous operationof ESM subarrays 46 is required, then each ESM subarray 46 may have itsown dedicated local network. Alternatively, the ESM subarrays 46 couldbe switched to a common local network. Antenna elements 36 used withinthe ESM subarray 46 are also used within TTD subarrays 14 and hencewithin digital subarrays 18.

The multifunction beam-forming network 50 will now be described infurther detail with reference to FIG. 4. The multifunction beam-formingnetwork 50 uses both local and remote networks and is used for allfunctions other than the ESM analysis function. FIG. 4 shows in detailthe multifunction beam-forming network 50 as it is used to drive anantenna element 36 in the right antenna array 28. It will be readilyunderstood that the antenna element 36 has been chosen arbitrarily andthe figure is equally applicable to all antenna elements 36 within theright antenna array 28 (excluding the ESM elements 44 that are not partof the antenna array 28 proper). Moreover, choice of the right array 28is also arbitrary: all antenna arrays 26, 28, 30, 32, 34 are equivalentin terms of the structure illustrated in FIG. 4. To illustrate where theremaining antenna arrays feed 26, 30, 32, 34 into the multifunctionbeam-forming network 50 of FIG. 4, the left array is indicated at 26′.The top 30, bottom 32 and rear 34 arrays also feed in at this point, buthave been omitted from FIG. 4 for the sake of clarity. The alternativemode of operation, i.e. ESM multiple beam indicated at 52 and will bedescribed later with reference to FIG. 5.

Returning to the multifunction beam-forming network 50 of FIG. 4, eachantenna element 36 can operate with a choice of two orthogonalpolarisations 36 a,b. For each polarisation 36 a,b, there is provided atransmit/receive switch 54. In the embodiment of FIG. 4, the switch 54is a circulator: however, in applications where these devices provideinsufficient isolation, they could be replaced by a two-way switch. Onthe receive side, the circuit for each polarisation 36 a,b is divided toprovide inputs into both the multifunction 50 and multiple beam 52beam-forming networks, with selection being made via the two way switchat 56.

Each polarisation 36 a,b also has its own amplifiers 58. This allows thebeam-forming to be achieved on the low power side of the amplifiers 58and also doubles the available transmit power. The two polarisations 36a,b are combined/separated via a double hybrid network 60. The pathlength adjuster 62 compensates the time delay between the two antennaelements' polarisation phase centres. The phase adjuster 64 controls thepower division between the two polarisations 36 a,b. Regulating thesetwo devices allows the polarisation state 36 a,b of an antenna element36 to be controlled according to a specified direction. Thispolarisation 36 a,b can be horizontal, vertical, slant linear, right orleft circular, or right or left elliptical.

Only one port of the double hybrid network 60 is fed into the localbeam-forming network. The unused port is loaded, as shown at 66. Thismeans that only one of the antenna element's polarisations 36 a,b can beaccessed at any one time. However, the loaded port 66 could be used toprovide an orthogonal polarisation state where there is a desire to useboth polarisations 36 a,b simultaneously. To allow this, the loaded port66 may be connected to a duplicate local beam-forming network.

The antenna element 36 is then connected to the TTD network 68, as areall other antenna elements within its TTD subarray 14, via a variableattenuator 70. Provision of amplitude and phase control allows timedelays to be applied at this TTD subarray level. This is advantageousbecause true time delay is required for the wider instantaneousbandwidth applications.

Whilst it is preferred to use the path length adjuster 62, the phaseadjuster 64 and the variable attenuator 70 together as part of a doublehybrid network 60, it is not outside the scope of the invention for anycombination of these components to be used, either in or out of thecontext of a double hybrid network 60.

When in receive mode, the output from each antenna element 36 within theTTD subarray 14 is combined by the TTD network 68 to produce an outputthat is passed via a pair of switches 72 to a laser diode 78 for upconversion to an optical carrier frequency which is then sent via anoptical fibre link 76 to the multifunction remote beam-forming network16. Conversely, when in transmit mode, an optical signal from themultifunction remote beam-forming network 16 is down-converted by aphotodetector 74 before being passed to the TTD network 68 via theswitches 72 for separation and onward transmission to the appropriateantenna elements 36 within the TTD subarray 14. By using an opticalfibre link 76, the remaining beam-forming network components can behoused at a remote location.

The left, top, bottom and rear antenna arrays 26, 30, 32, 34 would alluse similar local beam-forming networks to that shown in FIG. 4. Aswitch 82 is shown prior to the digital network 80 to switch between TTDsubarrays 14 from the left and right antenna arrays 26, 28. The top,bottom and rear antenna arrays 30, 32, 34 have been omitted from FIG. 4for the sake of clarity but it will be readily understood that their TTDsubarrays 14 would be connected to the network through the switch 82 inthe same way as for the TTD subarray 14 of the right antenna array 28.The position of the switch 82 is purely a matter of choice. The switch82 may be positioned close to the multifunction remote beam-formingnetwork 16 or it may be positioned closer to the antenna arrays 26, 28,30, 32, 34 (remembering that the TTD networks 68 are part of the localnetworks 12). The latter arrangement may be beneficial where a clearreduction in total optical path length may be achieved—this isforeseeable due to the reduction in the number of optical fibre links.

True time delay is provided in the optical domain at the multifunctionremote beam-forming network level using a binary fibre optic delay line(BIFODEL) 84. Groups of TTD subarrays 14 are combined into a digitalsubarray 18. The digital subarray 18 is combined by the digital network80 that is, in turn, connected via a switch 86 to either a photodetector88 for down conversion to RF (or intermediate frequency on receive) orto a laser diode 90 on transmit.

The multifunction remote beam-forming network 16 will now be consideredfor the receive path. A wide-bandwidth receiver 20 is provided for eachdigital subarray 18. The outputs are passed through a filter 92appropriate for the required function via a pair of switch matrices 94.The resulting signals are then converted to digits by the A/D device 22.The bandwidth of the signals are limited to that required for theparticular function so that the required speed of the A/D device 22 canbe reduced. This allows the A/D device 22 to cover a higher dynamicrange with increased accuracy.

The digital signal processor (DSP) 24 combines the outputs derived fromthe different digital subarrays 18 via the digital network 80 to formthe required beams. Simultaneous beams using digital subarrays 18 thatcover the whole left or right arrays 26, 28 can be produced providedthey are in the same general direction. For example, with appropriatedesign of the digital subarray 18 configuration, low sidelobe sum,azimuth difference and elevation difference beams can be generated. Forlower gain beams, the antenna arrays 26, 28, 30, 32, 34 can besubdivided into smaller digital subarrays 18, each of which can becontrolled independently to form beams in different directions eitherfrom an antenna array or from different digital subarray groups in thesame antenna array or different antenna arrays. If sufficient isolationcould be provided, two or more antenna arrays 26, 28, 30, 32, 34 couldalso produce simultaneous transmit and receive beams. The use ofopposite antenna array sides would offer higher isolation for this task.The use of such techniques should aid the time management of the variousmodes of operation required by the functions. The goal is to allow allfunctions to be usable without the need for the expensive parallelbeam-forming networks that would normally be required for simultaneousbeam formation.

Assuming that the number of digital subarrays 18 in the left and rightantenna arrays 26, 28 is the same and that the number of digitalsubarrays 18 is equal in the top and bottom arrays 30, 32, then thenumber of ports into the DSP 24 could be equal to the sum of the digitalsubarrays 18 in the left and top antenna arrays 26,30. This simplifiedconfiguration places a restriction that only one of a pair ofcomplementary digital subarrays 18 in the left or right antenna arrays26, 28 (or top or bottom antenna arrays 30, 32) can be used at any time.

Increased flexibility can be introduced at the expense of cost byproviding a greater number of inputs into the DSP 24 along with a moreflexible switching arrangement that allows different combinations ofdigital subarrays 18 to be used simultaneously. In the extreme case, alldigital subarrays 18 from all the antenna arrays 26, 28, 30, 32, 34would have an independent route into the DSP 24. However this wouldrequire duplication of all the equipment beyond the multiway switch 82.

Adaptive signal processing can be applied at the digital subarray levelat 24 to any of the receive beams that need to be formed.

The multifunction remote beam-forming network 50 will now be consideredfor the transmit path. The requirements for the transmit beams are farless demanding than for receive and will not generally require adaptivebeam control. This means that a more conventional beam-forming networkof the type well known in the art may be used. Such a conventionalbeam-forming network is described by M I Skolnik in Chapter 11.7 (‘FeedNetworks for Phased Arrays’) of The Radar Handbook, published by theMcGraw-Hill Book Company. However, the use of a DSP 24 on transmitallows the same high degree of flexibility as achievable on receive. Ifthis was implemented, the transmit path would be similar to the receivepath with the use of D/A devices and the DSP 24 to form the transmitbeams.

The ESM multiple beam beam-forming network 52 will now be described withreference to FIG. 5. For the ESM analysis mode, a fan of simultaneousbeams are required in one plane so as:

-   to act as a spatial discriminator and indicate the direction of the    threat;-   to increase the signal to noise; and-   to reduce the amount of data that must be processed in a single ESM    channel.

FIG. 5 shows a layout for an antenna array 26, 28, 30, 32, 34 of antennaelements 36 with dual polarisations 36 a,b that have separate phasecentres (e.g. Vivaldi elements). The ESM subarray consists of antennaelements 36 disposed along a line. If required, antenna elements 36could be combined in the perpendicular plane to increase directivity inthis plane.

As for FIG. 4, FIG. 5 shows an arbitrary antenna element 36 from theright antenna array 28. The figure represents equally well other antennaelements 36, both from the right antenna array 28 and from the otherantenna arrays 26, 30, 32, 34.

FIG. 5 shows that the output from each polarisation 36 a,b of theantenna element 36 is split between the ESM multiple beam beam-formingnetwork 52 and the multifunction beam-forming network 50, as is alsoshown in FIG. 4. Each polarisation 36 a,b has a laser diode 98 that isused to upconvert the RF received by the antenna element 36 to anoptical carrier frequency. This is the extent of the ESM local network40 because the optical signal is then passed via an optical fibre 100 tothe remote multiple beam beam-forming network 42.

The multiway switch that allows signals from the remaining antennaarrays 26, 30, 32, 34 to be passed to the remote multiple beambeam-forming network 42 is shown at 102 for each polarisation 36 a,b.For the sake of clarity, only the left antenna array 26 is shownalthough there is provision for switching between left, right, top,bottom and rear antenna arrays 26, 28, 30, 32, 34 .

Each antenna element 36 feeds a pair of signals, according topolarisation, into the remote multiple beam beam-forming network 42.This is an optical beam-former that forms a simultaneous fan of pencilbeams in one plane. The remote multiple beam beam-forming network 42performs a similar function to the well-known Rotman Lens. In fact,remote multiple beam beam-forming network 42 is of standard design andso will not be described further here. An example of such a beam-formingnetwork is provided in True Time Delay Beamforming Using Fibre OpticDelay Lines by Cortis and Sharpe (IEEE AP-S International SymposiumDigest, 1990, pages 758 to 761).

As will be readily evident, variations to the above embodiment arepossible without departing from the scope of the invention. Someexamples of possible alternatives have been noted in the descriptionabove.

1. An antenna system, comprising a beam-forming network linked to aplurality of antenna elements organized, in operation, into a pluralityof first-order groups of antenna elements, the beam-forming networkcomprising: a plurality of local networks, each of said local networksfor manipulating signals received by or to be transmitted by antennaelements of at least one of said plurality of first-order groups ofantenna elements; a common remote network for manipulating signalsreceived from or to be transmitted to said plurality of local networks;and a controller for dynamically reconfiguring the organization ofgroups from said plurality of first-order groups of antenna elementsinto at least one second-order groups of antenna elements.
 2. An antennasystem according to claim 1, wherein the organization of antennaelements into said plurality of first-order groups is fixed.
 3. Anantenna system according to claim 1, wherein said controller is operablefurther to select a frequency band for receiving signals from ortransmitting signals to said at least one second-order group of antennaelements.
 4. An antenna system according to claim 1, wherein each ofsaid plurality of local networks is operable to combine signals receivedfrom the antenna elements of a respective first-order group beforetransmission to the remote network and to separate a signal receivedfrom the remote network into signals for transmission to the antennaelements of a respective first-order group.
 5. An antenna systemaccording to claim 1, wherein the local network is operable with RFsignals.
 6. An antenna system according to claim 5, wherein the remotenetwork is operable with optical frequency signals.
 7. An antenna systemaccording to claim 6, wherein the local network is operable to upconvertan RF signal to an optical frequency signal prior to transmission to theremote network.
 8. An antenna system according to claim 1, wherein theremote network is operable to digitize a signal received from the localnetwork.
 9. An antenna system according to claim 1, wherein the remotenetwork provides true time delay to signals received from or fortransmission to said plurality of local networks.
 10. An antenna systemaccording to claim 1, wherein said antenna elements are operable withone of two polarizations.
 11. An antenna system according to claim 1,wherein the polarizations are mutually orthogonal.
 12. An antenna systemaccording to claim 1, wherein each of said at least one second ordergroup is provided with a receiver.
 13. An antenna system according toclaim 1, further comprising at least one group of antenna elements foruse in an ESM analysis mode.
 14. An antenna system according to claim13, comprising a further beam-forming network operable to receivesignals from the antenna elements of said at least one group of antennaelements for use in an ESM analysis mode.
 15. An antenna-systemaccording to claim 14, wherein the further beam-forming networkcomprises a local network and a remote network.
 16. An antenna systemaccording to claim 1, further comprising ESM antenna elements fortransmission of ESM signals.
 17. A platform comprising at least oneantenna system according to claim
 1. 18. A platform according to claim17, wherein the platform is an airborne vehicle, a ship or a boat.